There are many bonded materials deployed in structural arrangements wherein the structural utility of the bonded material depends on a bond quality of the bonded material. In applications where the structural arrangements are critical, there is a need to evaluate the bond quality.
Honeycomb-structured materials allow fabrication of structures having reduced weight while keeping a very high stiffness. Such materials are particularly useful in the aeronautics and space industry. Honeycomb structured materials typically have two skins, and a core layer of ribs extending perpendicular to the skins to form hexagonal cells. The skins are usually laminates, such as e.g. carbon-epoxy laminates made of layers of carbon fibers in an epoxy resin. Two kinds of disbonds could occur in these structures, both resulting in a weakened structure: detachments between one skin and the ribs of the honeycomb; and delaminations within the skin. Probing honeycomb-structured components to find any disbond is important for assessing the quality of newly produced parts, and a condition of components during service. Similar structures also widely used in the aeronautics and space industries include a foam material intermediate the two skins instead of a honeycomb core layer.
Coated and laminated materials are other examples of materials for which bond assessment is important. Coatings are widely used on industrial material surfaces for protection against wear, oxidation, and corrosion or as thermal barriers, for example. Voids or detachments at the coating-substrate interface result in a fragile coating that could become detached, leaving the substrate unprotected and subjected to severe heat load, erosion, oxidation or corrosion. Coatings are made by different methods such as electroplating, thermal spray, painting, etc. or vacuum deposition for the thin and ultra thin multilayer coatings used by the microelectronics industry.
Very few techniques are known that can be applied to honeycomb or foam core-structured materials, particularly if the structure can only be interrogated from only one side. Known techniques use penetration of heat and/or acoustic waves (i.e. transmission ultrasonics) to detect disbond between a skin and the core. Difficulties arise because of limitations of penetration of acoustic and thermal waves through the bonded structure, and consequently there is very little change of properties, such as acoustic reflection or thermal conductivity, between a good bond and lack of bond. In the case of honeycombs this is partially due to the thinness of the ribs and in the case of foams to their very high porosity, which are the very properties that provide the strength and lightness that make the structures commercially valuable. Most of the techniques that have been developed to test bond integrity of coatings and the integrity of laminates from single side access cannot be applied reliably to honeycomb or foam core structured materials.
In practice, honeycomb and foam-core structures are presently inspected using transmission ultrasonics, i.e. with an emitting transducer on one side of the part and a receiving transducer on the other side (usually coupled with water jets on both sides). This technique, which requires access to both sides of the material, is possible at fabrication but usually not possible while assembled in a structure (such as in an airplane wing).
Regarding coatings, many coatings used in industry highly attenuate ultrasound. This is the case of thermal barrier coatings used on turbine blades and silicon carbide protective layers on carbon-carbon composites. The ultrasonic wave is strongly scattered and a very small coherent ultrasonic signal returns to the material surface in accordance with the pulse echo configuration. This large attenuation can be traced back to the size of material microstructure or porosity compared to the ultrasonic wavelength. For very thin coatings, attenuation does not damp out the coherent signal, but very high ultrasonic frequencies are required to resolve the ultrasonic echoes reflected back from the coating-substrate interface. Therefore all the existing ultrasonic techniques that require either contact or water coupling or non contact as in laser-ultrasonics have important limitations.
U.S. Pat. No. 4,752,140 to Cielo entitled “Pulsed dilatometric method and device for the detection of delaminations”, proposes local heating and local interferometric detection. As explained by Cielo in U.S. Pat. No. 4,752,140 when laser heating is not uniform and is concentrated over an area smaller than the size of the detached zone, localized thermal stresses are produced that cause a stronger lifting and bending effect. As explained further by Cielo in various publications and in particular in “Thermoelastic Inspection of Layered Materials: Dynamic Analysis” (Materials Evaluation, vol. 43, pp. 1111-1116, 1985), the disbonded layer or skin can then be set into vibration like a membrane.
This latter approach proposed by Cielo has, however, several shortcomings which explain why it has not found practical use in industry despite having been disclosed more than 20 years ago. One is the fact that Cielo uses a Michelson-type interferometer (homodyne or heterodyne) for detection. When a high intensity light beam strikes a surface, typically the beam is reflected in all directions in an uneven manner defining a speckle pattern, each speckle of which having a high intensity. Homodyne or heterodyne interferometers are sensitive to the optical speckle produced by the roughness of the surface, which means that these interferometers have a maximum sensitivity to surface displacement when only one speckle of the light scattered by the surface is collected. This is because each speckle of light returning from the surface effectively is a separate beam having its own respective random phase offset and random amplitude, and consequently, if multiple speckles are gathered for use in such an interferometer, each pair of beams (speckles as well as the reference beam) produces a separate interference pattern, all of which interference patterns are superimposed, resulting in a sharply reduced sensitivity of the measurement.
Since the intensity of the collected speckle typically varies strongly from one interrogation location on the surface to the other, the sensitivity of the device also strongly varies from one location to the other. Accordingly, scanning a part to get an image of the adhesion integrity of the structured material is not very practical.
This technique can be applied to a honeycomb structure with specially polished aluminum skins for which the light reflected off the surface is nearly speckle-free. Such cases in practice never occur; skins are usually made not of aluminum but of polymer-matrix fiber reinforced laminates. It would be unpractical and too onerous to apply a special coating or paint which has a sufficiently smooth surface to give speckle-free reflection.
There therefore remains a need for a nondestructive technique for detecting a disbond in a bonded structure, such as a coated, honeycomb or foam-core structured material, where the disbond forms a membrane at a top layer of the bonded structure.